Morning!
I just want to offer you my compassion and empathy.
I have been where you are more times than I could count. In my case, I had a problem with alcohol. I was using alcohol as a crutch when my mental health was in the toilet. I wasn't drinking daily, but when I did drink, I drank hard and fast and blacked out often. Sometimes I was with people, other times I was alone. (I'm now happy and sober and in a good place mentally )
I don't forget that feeling : shooting blot up right as the dawn breaks with a panic attack induced gasp. Mouth as dry as a desert, head pounding, limbs aching, a tiredness that's all consuming, but a brain that's on fire with activity.
My obsessions and paranoia lost black out were like yours. My two themes were: thinking I'd had sex with someone (even worse feeling if I was in a relationship) and could now be pregnant or have an STD. I'd have to neck some wine before checking my phone for any unknown numbers or suggestive text messages.
The second one : was getting in my car. I was utterly petrified that I may have driven my car whilst drunk. Did I crash it ? Have I killed someone? I'd keep all the blinds closed, not daring to look out the window to see if my car was there or not or if it was parked in a chaotic way. I'd have to neck a glug or 7 of wine before I went out there, sweating, to see if there was any blood spats or dents.
The fear, terror and anxiety that provoked was like nothing in this world. My heart is pounding just sitting there writing it.
To reassure you: the times when I had indeed fucked up and done something magnitudously regretful whilst black out drunk - I'd get flashbacks. Laughing with some random in a bar. The smell of aftershave. A flashback of writing out some weird text and a resurface of whatever emotion I was feeling at the time. As PPs have said, if it's significant enough - you will very likely get flashbacks or obvious clues.
What you're doing at the moment Is confirmation bias. You've convinced yourself you've slept with someone and every twinge or ache you're attributing to it.
I honestly believe you're fine, I really do. Especially if you weren't even chatting to any bloke before hand. I would put a large chunk of money on the fact that you walked home in your weary state and fell into bed. There'll be no more to it. So forgive yourself and move on my lovely.
There's a bit but. That but Is this: take a lesson from me, who wouldn't relieve those moments if someone instantly made me a billionaire. Stop drinking. This is a warning that alcohol and you don't see eye to eye. You really don't want to be in this position again.
I was already an anxious, overthinking, over analytical being. So the hang-xiety of a blackout almost spiralled me into suicidal ideation.
Good luck. Hope you're feeling calmer and more rational today x